System for assaying solutions containing alpha-emitting isotopes using track-registration material

ABSTRACT

A method for determining the quantity of an alpha-emitting ingredient in a solution. A sheet of track-registration material is brought into close proximity to the solution, held there for given period of time, and then removed. This material has the characteristic of forming damage tracks along paths traversed by alpha particles. These paths may be enlarged and made visible by treatment with a suitable etchant which enlarges the tracks. The number of tracks formed in a given area is an indication of the quantity of the alpha-emitting ingredient present in the solution.

United States Patent Inventor Henry Ward Alter Danville, Calif. 708,255

Feb. 26, 1968 Feb. 9, 1971 General Electric Company Appl. No. Filed Patented Assignee SYSTEM FOR ASSAYING SOLUTIONS CONTAINING ALPHA-EMITTING ISOTOPES USING TRACK-REGISTRATION MATERIAL 7 Claims, 2 Drawing Figs.

US. Cl. 250/83 Int. Cl. G0lt l/04 Field of Search 250/83 I References Cited OTHER REFERENCES Fleischer et al.; Tracks of charged Particles in Solids SClENCE; 23 July 1965, Vol. 149, 03682 250/83cd Primary Examiner-Archie R. Borchelt Assistant Examiner-Morton J. F rome Attorneys-Ivor J. James, Jr., Samuel E. Turner, John K Duncan, Frank L. Neuhauser, Oscar B. Waddell and Melvin M. Goldenberg PREPARE TRACK- REGISTRATION SHEET PLACE SHEET IN THIN IMPERM EABLE BAG 0mm SOLUTION oF ALPl-IA EMITTER REMOVE FROM SOLUTION w REMOVE SHEET FROM BAG ETC H RINSE COUNT TRACKS SYSTEM FOR ASSAYWG SOLUTKONS (IONTABNENG ALFHA-ENETTTNG ESOTOIPES USING TRACK- .REGISTRATION MATERlAL BACKGROUND OF THE INVENTION Many radioactive isotopes which decay by the emission of alpha particles are of considerable interest in nuclear science and engineering.

Modern power reactors may use a variety of fissionable isotopes in the fuel. Typical isotopes include U--233, U-235 and Pu-239. Such reactors are described in detail, for example, in US. Pat. No. 2,708,656 to Fermi et al. and US. Pat. No. 3,029,l7 to Untermyer. in reactors of the breeder-converter type, such as are described in US. Pat. No. 3,211,621, Pit-239 is especially desirable because. of its uniquely high yield of approximately three neutrons per Pu-239 fission. Thus, fuel for breeder-converter reactors often consists of mixtures of Pu-239 and other materials, such as U-235and U-238. Often, the final solid fuel is precipitated from a solution of the several ingredients. The continuous maintenance of the desired proportions of Pu-239 and other ingredients is of extreme importance in order that the fuel have uniform and consistent nuclear characteristics. It is, therefore, highly desirable that the relative proportions of the ingredients be monitored during fuel fabrication. Decay of Pu-239 is by alpha particle emission, as is the decay of U-235 and U-238. While the half-life of Pu-239 is rather long, i.e. 24, 360 years, the half-lives of U-235 and U-238 are about 30,000 and 200,000 times longer. Thus, the alpha emission from a solution containing these isotopes is'for all practical purposes entirely produced by the Pu-239 where a reasonable proportion of the mixture is Pu-239. Thus, a solution including these isotopes, and any other material which is not an appreciable alpha-emitter, may be assayed for Pu-239 by measuring alpha emission over a given time period.

Similarly, Pu-238 has been found to be useful, for example, in various thermionic power sources, such as is described in U.S. Pat. No. 3,330,974. Since Pu-238 is an alpha-emitter, the quantity of Pu-238 in a nonalpha-emitting matrix may be determined by measuring aplha emission of the mixture.

Medical uses have been found for a great many radioisotopes, many of which decay by the emission of alpha particles. The radioisotope is generally mixed in very low concentrations with an inert carrier. It is obviously of extreme importance that the radioisotope concentration be known and constantly regulated in the manufacture of these materials.

Several techniques have been used in the past to measure alpha activity in materials. Various kinds of electronic instrumentation have been used to detect and-measure alpha radiation. Such equipment is expensive, bulky, usually requires trained operators and often has low sensitivity. Radiochemical preparation of samples is usually required. Photographic film has also been used to detect alpha radiation because the lightsensitive emulsion of the film will record tracks of alpha particles. Such film, however, must be packaged in lighttight containers and must be processed using photographic darkroom techniques. Photographic film is thus both inconvenient and relatively expensive to use.

Thus, there exists a continuing need for a simple and inexpensive but sensitive method for measuring alpha radiation inchemical solutions.

SUMMARY OF THE INVENTION It is, therefore, an object of this invention to provide a method for measuring alpha radiation in solutions which overcomes the above-noted problems.

Another object of this invention is to provide an improved method for measuring the quantity of an alpha particle emitting ingredient in a solution.

The above objects, and others, are accomplished in accordance with this invention by exposing a sheet of track-registration material to the solution containing an alphaernitting ingredient for a given period of time, whereby damage tracks are formed by the penetration of alpha particles into and/or through the material, then etching the tracks with a suitable reagent to make them visible. The number of tracks in a given area is a direct indication of the quantity of alpha-emitting material in the solution. A count of the number of tracks in the given area then may be compared to a standard count for the alpha-emitting material to thus determine its concentration in the solution.

BRIEF DESCRIPTION OF TI'E DRAWING Details of the invention will be further understood upon reference to the drawing, wherein:

FIG. 1 shows a flow sheet for a preferred technique of assaying a solution for the concentration of an alpha-emitter present therein; and

FIG. 2 shows a simple device which is especially useful in carrying out the method shown in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION Any suitable material may be used in the track-registration sheets. Typical materials include crystalline solids such as mica or a noncrystalline material such as inorganic glass or an organic polymer. If the material is subjected to irradiation with charged particles, minute damage tracks are created in the material by local alteration of the material structure along the particle trajectories. The damage tracks may be enlarged and made visible by application of a reagent which preferentially attacks the altered material at a faster rate than it attacks the unaltered material around the track. This tracketch process is further described in U.S Pat. No. 3,303,085 to Price et a].

Certain track-registration materials, such as cellulosic polymers including cellulose nitrate cellulose acetate, and cellulose acetate butyrate, register damage tracks when subjected to alpha radiation, but do not register beta and gamma radiation damage tracks. Cellulose nitrate is a preferred material since it produces excellent tracks, is inexpensive and is readily available.

Any alpha tracks which may have been formed in the material prior to intentional exposure from background radiation normally present in the air may be erased by heating the track-registration sheet to its softening temperature whereby the latent tracks are removed. Alternatively, fresh film may be cast from a solvent. Generally, however, the tracks produced by background radiation are so few compared to those produced by exposure to the solution that eliminating this preparatory step does not introduce appreciable inaccuracies into the measurement.

Any suitable etchant may be usedto enlarge the damage tracks to a size suitable for counting. Typical etchants include sodium hydroxide, potassium hydroxide, ammonium hydroxide, sodium carbonate, and mixtures thereof. Of these, best results are obtained with about six-normal sodium hydroxide.

The sheet may be treated with the etching solution for any suitable time period. Where the period is too short, the tracks will not be readily visible, while too long a treating period allows the solution to attack the body of the sheet excessively. The temperature of the treating solution has a major effect on rapidity of etching. Typically, a cellulose nitrate track-registration sheet treated with six-normal sodium hydroxide gives best results in about 35 minutes at about 40 C. At room temperature, several hours may be required to give similar results.

It is necessary that the track-registration material be positioned in close proximity to the source of alpha radiation, since the range of alpha particles in air and various liquids is very short. For example, the alpha particles emitted by Pu-239 have an energy of about 5.15 Mev. and a range in water of about 4 X 10 centimeters.

While it is possible to dip the track-registration material directly into the solution to be assayed, this is generally undesirable. When the material is removed from the the solution,

some of the alpha-emitting ingredient may remain on the material. Even if the material is carefully washed, some contamination may remain. Thus, alpha tracks may be produced after the intended exposure time has ended, resulting in inaccurate measurement. Also, the contaminants may be extremely hazardous to personnel. This will require careful handling of the material duri ngetching and counting.

It is, therefore, strongly preferred that the track-registration material be isolated from but in close proximity to the solution during exposure.

Referring now to FIG. 1, there is seen a simplified flowsheet for a especially preferred assaying procedure within the general method of this invention.

First, a sheet of track-registration material, such as is described above, is prepared as indicated in block 10. The sheet may have any desired thickness. If the sheet is very thin, less than about 20 microns in thickness, substantially all of the alpha particles which impinge the sheet will pass through since the average range of alpha particles in these materials is about 40 microns. Thus, counting the openings on one side will give substantially the total alpha particles which hit the sheet on both sides. On the other hand, if the sheet has a thickness of greater than about 50 microns, substantially none of the alpha particles will penetrate entirely through the sheet. For ease of handling, this sheet may have a thickness on the order of 0.1 inch. Here, a count of the track openings on one side of the sheet will only count the alpha particles which struck that side of the sheet.

This especially desirable technique uses a bag such as is shown in FIG. 2 at 100 of a very thin solution-impermeable material, such as polyethylene. Preferably, the bag has a wall thickness of about microns. A weight 101 is placed in the bottom of the bag, then the sheet of track registration material 102 is inserted into the bag as indicated in block 11 in FIG. 1. The assembly is then dipped into the solution to be assayed as indicated in block 12. The weight causes the bag to assume a vertical position, so that the solution forces air out of the bag and the bag walls collapse tightly around the sheet of track registration material. The bag is maintained in place for the desired period, then it is removed from the solution as indicated in block 13. The sheet is removed as indicated in block 14 and the bag is disposed of. The sheet is completely uncontaminated. There is no residual exposure problem once the sheet is removed from the bag.

The sheet is then etched and rinsed with water as indicated in blocks 15 and 16 and the number of tracks per unit area counted as indicated in block 17. This will indicate the concentration of alpha-emitting material present, based on previous quantitative calibration tests with known solutions and a qualitative knowledge of the ingredients in the tested solution.

Other techniques may be used to expose the track-registration sheet to alpha radiation from a solution to be assayed, if desired. For example, where the vessel containing the solution has a very thin wall, the track-registration sheet may be held tightly against the outer wall surface for the desired exposure period.

Generally, analysis of a solution is most accurate where only one alpha-emitter is present, or where there is a difference in alpha activity of at least about 10 between the most active alpha emitter and any others present. However, where two alpha emitters are present, it is possible to measure the quantity of the one producing higher energy alpha particles by covering the track-etch sheet with a film having sufficient thickness to stop the lower energy alpha particles from the second emitting material, while allowing higher energy alpha particles to penetrate to the track-registration sheet.

The expected number of tracks produced by a given alphaemitter in a given solution may be approximately calculated using the equation:

where T tracks/cm.2-min.; P P alpha d/cm.3-min. min. and D, sensitive alpha range in the sheet in centimeters. For

example, the tracks produced by an aqueous solution of Pu239 may be found as follows: The sensitive range of alpha particles in cellulose nitrate is about 20 microns or about 2 X 10- cm. At about 1 gram per liter, the alpha activity of Pu239 is about 1.4 X 10 disintegrations/cm. min. Thus:

T 0.7 X l0 /cm. -min.-g./l.

This factor may then be used to obtain the concentration of Pu239 in another solution merely by exposing a track-registration sheet for a given period to the solution and counting the resulting tracks in a given area.

Where the track-registration film is separated from the solution by a film, as in the above-described preferred embodiment, some of the alpha particles will be stopped in the film and not reach the track-registration sheet. In this case, it may be preferred to expose sheets for standard periods to a plurality of known solutions of different concentrations and plot a curve of the tracks obtained against concentration. Unknown solutions of the same alpha-emitter may be assayed by exposing a track registration sheet for the standard time, then obtaining the concentration from the curve.

Details of the invention will be further understood upon reference to the following examples, which point out various preferred embodiments of the method of this invention. Parts and percentages are by weight unless otherwise indicated.

EXAMPLE I Six aqueous solutions containing Pu239 are prepared having the following Pu239 concentrations: (a) l gram/liter, (b) 0.1 gram/liter, (c) 0.01 gram/liter, (d) 0.001 gram/liter, (e) 0.0001 gram/liter and (f) 0.00001 gram/liter.

Six sheets of cellulose nitrate track-registration material are then prepared. Each sheet is about 1 inch by 2 inches and has a thickness of about 0.1 inch. A few lead shot are placed in each of six polyethylene bags, each having an average Wall thickness of about 10 microns. A track-registration sheet is placed in each bag, and one of these assemblies is dipped into each of the above solutions. The solutions force air out of the bags and press the bags tightly against the sheets. The opening in each bag is kept above the solution surface so that no solution contacts the sheets. The sheets are left immersed in the solutions for about 16 hours. Then the bags are lifted out of the solutions, the sheets are removed and the bags are discarded.

The sheets, which bear damage tracks proportional to the concentration of Pu239 alpha-emitter in each solution, may be developed at any time. The sheets are not sensitive to beta or gamma radiation or light and are not easily damaged. The damage tracks are not at this time visible under an optical microscope, although they may be detected by electron microscopy.

The sheets are developed by placing them in a six-normal sodium hydroxide solution for about 35 minutes at about 40 C. The sheets are then water-rinsed to stop the etching action. The now-enlarged damage tracks can now be easily viewed using a 300 power optical microscope.

The number of tracks in an area measuring about 0.05 mm. is counted. Although the tracks are rather uniformly located over the sheet surface, several areas may be measured and a more accurate average number obtained.

The approximate number of tracks found in the 0.05 mm. field on each sheet is as follows: (a) about 34,000, (b) about 3.400, (c) about 340, (d) about 34, and (e) about 3. Plotting these results on coordinates of Pu239 concentration versus number of tracks gives a substantially straight line. Repeating the exposure and development steps with a Pu239 solution of unknown concentration will give a number of tracks which can be plotted against the above-developed curve to give Pu239 concentration.

As can be seen from these results, this technique is capable of accurately measuring very small concentrations of alpha emitters. This process would also not-be adversely affected by the presence in the solution of gross amounts of gamma or beta emitters. Where a very large number of tracks are obtained, as with solution a above, it would be better to repeat the measurement with a shorter exposure time so that the number of tracks may be more easily and accurately counted. Conversely, where very few tracks are produced, the exposure time may be increased.

EXAMPLE ll More concentrated solutions of alpha-emitters than those described in Example I may be measured by using shorter exposure times. Three solutions comprising Pu-239 in water are prepared with the following Pu-239 concentrations: (a) 1 gram/liter, (b)'l0 grams/liter, and (c) 100 grams/liter.

A thin polyethylene bag is prepared as described in Example 1. into each bag is inserted a 1 inch by 2 inch piece of 0.02 inch thick cellulose acetate track-registration material.

The bags are dipped into the solutions for about 1 minute, then are removed. The sheets are removed and the bags are discarded. The sheets are developed by immersing them in a five-normal solution of potassium carbonate at room temperature for about 5 hours. 7

Under a 400 power microscope, the etched tracks are readily visible. While the tracks are not quite as sharp as in Example l, they may easily be counted. The number of tracks visible on one side of each sheet in an area of about 0.05 mm. is as follows: (a) about 35 tracks, (b) about 35 tracks, and (c) about 3,500 tracks.

Thus, a direct relationship is seen between Pu-239 concentration and number of tracks per unit area. Also, since solutions a in both Examples l and 11 are identical, it can be seen that by reducing exposure from 16 hours to 1 minute, the l gram/liter solution produces about 35 easily counted tracks rather than about 34,000 difficult to count tracks in the microscope field.

Although specific components and proportions have been described in the above examples, other materials as listed above, may be used with similar results where suitable. In addition, other materials may be added to the track-registration materials and to the etching baths to synergize, enhance or otherwise modify their properties.

Other modifications and ramifications of the present invention will occur to those skilled in the'art upon reading the present disclosure. These are intended to be included within the scope of this invention.

lclairn:

l. A method for measuring the concentration of an alphaemitting ingredient in a solution which comprises positioning a track-registration material in close proximity to the solution and within the transit range of alpha particles in the solution, whereby the material is exposed to alpha particles emitted in the solution, said material being substantially insensitive to light, beta radiation and gamma radiation, and having the property of forming damage tracks along paths in said material transversed by alpha particles; etching said material after exposure with a reagent which selectively attacks and enlarges the damage tracks whereby the tracks are made sufficiently visible to be counted; making a count of the number of said tracks per unit area; and comparing said count to a standard count for said alpha-emitting ingredient to determine said concentration of said alpha-emitting ingredient.

2. The method of claim 1 wherein saidmaterial is positioned in close proximity to said solution byenclosing said material in a solution-impermeable film which is substantially transparent to alpha-radiation and dipping the enclosed material in said solution.

3. The method of claim 2 wherein said film forms a bag having an open top extending above the solution surface which permits air to be forced out of the bag by the solution whereby the balglis pressed ti htly against said material.

4. e method 0 claim 1 wherein said material comprises cellulose nitrate.

5. The method of claim 4 wherein said material is in the form of a sheet having a thickness of at least about 50 microns.

6. The method of claim 1 wherein said material is etched in a solution comprising sodium hydroxide.

7. An article for measuring the concentration of an alphaparticle emitting ingredient in a solution which comprises:

a. an open-topped bag of a material which is impermeable to said solution but permeable to alpha-particles, said bag having an average wall thickness of less than about 20 microns;

b. within said bag, at the closed bottom thereof, a member having a density greater than that of said solution; and

c. within said bag a sheet of track-registration material; said material being substantially insensitive to light, beta and gamma radiation and having the property of forming damage tracks along paths in said material traversed by alpha particles. i 

2. The method of claim 1 wherein said material is positioned in close proximity to said solution by enclosing said material in a solution-impermeable film which is substantially transparent to alpha-radiation and dipping the enclosed material in said solution.
 3. The method of claim 2 wherein said film forms a bag having an open top extending above the solution surface which permits air to be forced out of the bag by the solution whereby the bag is pressed tightly against said material.
 4. The method of claim 1 wherein said material comprises cellulose nitrate.
 5. The method of claim 4 wherein said material is in the form of a sheet having a thickness of at least about 50 microns.
 6. The method of claim 1 wherein said material is etched in a solution comprising sodium hydroxide.
 7. An article for measuring the concentration of an alpha-particle emitting ingredient in a solution which comprises: a. an open-topped bag of a material which is impermeable to said solution but permeable to alpha-particles, said bag having an average wall thickness of less than about 20 microns; b. within said bag, at the closed bottom thereof, a member having a density greater than that of said solution; and c. within said bag a sheet of track-registration material; said material being substantially insensitive to light, beta and gamma radiation and having the property of forming damage tracks along paths in said material traversed by alpha particles. 